For PCBs that operate at high frequencies (i.e. about 25 GHz), blocking capacitors are often employed. These discrete capacitors are usually placed in the signal flow path to block unwanted direct current (DC) signals. While these discrete capacitors (or other discrete components, like resistors) may (and usually do) represent a small impedance discontinuities due to the relatively small value (i.e., 1 μF represents 0.2 mΩ at 1 GHz), the discrete component is physically much larger that the trace or transmission line to which it is coupled; this physical size can cause a capacitive discontinuity in the transmission line or trace, which can limit the frequency performance.
One example of a conventional solution for this capacitive discontinuity is to make adjustments to the ground plane within the PCB, as shown in FIGS. 1 and 2. In this example, the discrete component 106 (which can, for example, be a capacitor) is coupled to trace 108 that is formed on PCB 102. Within the PCB 102, there is a metal layer that forms a ground plane 104. In a region that underlies the discrete component 106, there is a window 110 that is formed in the metal layer 104. The hole or window 110 in the metal layer 104 decreases coupling to ground by trace 108 and increases the inductance to ground for the ground return current. This has the overall impact of raising the impedance of the discontinuity so as to achieve better matching.
There is, however, a drawback to this conventional solution. Namely, there is a frequency limit under which this solution will function. Thus, there is a need for an apparatus that allows matching at higher frequency ranges.
Some examples of conventional solutions are: U.S. Pat. No. 6,992,374; U.S. Pat. No. 7,623,353; and U.S. Patent Pre-Grant Publ. No. 2006/0044734.